1. Field of the Invention
The present invention relates to a thermal processing apparatus for the heat treatment to an object to be processed, such as a semiconductor wafer.
2. Description of the Related Art
A known vertical heat treatment apparatus, i.e., one of semiconductor device fabricating apparatuses, has a heat treatment furnace provided with a heater surrounding a vertical reaction tube. A wafer holder holding a plurality of wafers in a tier-like manner is carried into the heat treatment furnace from below the heat treatment furnace. The interior of the reaction tube is heated to a predetermined temperature to subject the wafers to a film forming process or an oxidation process.
The heater mainly consists of a heating element that is made of a metal such as an iron-chromium alloy or a ceramic material such as MoSi2. The heating element is formed in a helical shape so as to surround the reaction tube or in a wavy shape so as to extend along the circumference of the reaction tube. Different parts of the heat treatment furnace may radiate heat at different rates, respectively. Thus, the heater is divided into a plurality of heater sections, such as an upper, a middle and a lower heater sections. Then, temperature controllers are combined with the heater sections, respectively, in order to control the temperature of a processing atmosphere in such a manner that the widest possible region of the processing atmosphere can be heated in a highly uniform temperature distribution. Such a mode of control is called a zone control mode.
Recently, the diameter of the wafer has been progressively increasing. In addition, the thickness of thin films has been progressively decreasing for miniaturization of semiconductor devices. Therefore, the size of the heat treatment furnace has also been progressively increasing, while highly uniform temperature distribution has been required in the processing atmosphere. Such a requirement may be met by dividing the heater into an increased number of heater sections for an increased number of heating zones, and by individually controlling respective operations of the heater sections. When such a zone control mode is employed, however, the number of the controllers increases and hence cost increases. Furthermore, maintenance work, such as work for temperature calibration, is complicated and there are practical difficulties in such a zone control mode. Therefore, it is preferable to assign one temperature controller to a wide region and to minutely control the heat generation pattern (shape and heat generating rate) of the heater.
Mechanical strength of a conventional heating element becomes insufficient if the width (or the diameter) of the heating element is reduced, therefore, a heating element having great width is used unavoidably. A heating element of a great width cannot be bent in a curve or a small radius of curvature due to the great width and the properties of a material forming the heating element; that is, a degree of freedom in bending for shaping of the heating element is small. Even if it is intended to use a heating element having a plurality of sections that have different values of resistance to generate heat at different heat generating rates, respectively, it is difficult to process a workpiece to form a heating element partly having different diameters, and the diameters cannot be freely determined because the mechanical strength of the heating element is dependent on the diameter. Thus, the minute adjustment of heat generation pattern of the heater is difficult, and it is difficult to heat the processing atmosphere to a uniform temperature distribution.
A reaction tube made of quartz may become permeable to molecules when heated to a high temperature. Therefore, there is possibility that wafers are contaminated with impurities contained in the heating element, such as a metal or a ceramic material. In some cases, a reaction tube made of SiC is used in order to suppress the permeation of impurities through the reaction tube. However, the reaction tube made of SiC has a large heat capacity. Therefore, if a reaction tube made of SiC is employed, it is possible that a characteristic of temperature control of the processing atmosphere is deteriorated, a time necessary for stabilizing the temperature is increased and a throughput of the thermal processing is reduced.
The present invention has been made in view of those problems and it is therefore an object of the present invention to provide a thermal processing apparatus capable of controlling a heat generation pattern at a high degree of freedom and of forming a processing region having a highly uniform temperature distribution.
According to the present invention, the heating element can be formed in a desired shape, and the resistance-heating element may be of a desired diameter. Therefore, sectional regulation of heat generating rate can be easily achieved; that is, the degree of freedom of design of heat generating pattern is high. Consequently, a processing region can be heated in a highly uniform temperature distribution.
Preferably, the furnace body is a heat insulating body. In the case, it is preferable to provide a cooling medium passage for introducing a cooling medium in order to cool the furnace body. In addition, it is preferable that the heater is arranged apart from an inner surface of the furnace body.
Preferably, the reaction vessel has a shape of a longitudinal cylinder, and the heater is disposed in a region facing a side surface of the reaction vessel. In the case, further preferably, the heater has a longitudinal shape, and a plurality of sealing members are arranged in parallel with the heater vessel.
In addition, preferably, a second heater is disposed in a region facing a top wall of the reaction vessel, and the second heater includes heating elements, each having a sealing member made of a ceramic material, and a linear flexible resistance heat generating member sealed by the sealing member.
In addition, preferably, a third heater is disposed in a region near to a lower end of the reaction vessel, and the third heater includes heating elements, each having a sealing member made of a ceramic material, and a linear flexible resistance heat generating member sealed by the sealing member.
Preferably the furnace body has a mirror-finished inner surface. In the case, it is similarly preferable that the heater is arranged apart from an inner surface of the furnace body.
Alternatively, the furnace body may include: a first heat reflector having an inner surface serving as a heat reflecting surface, and a second heat reflector surrounding the first heat reflector, having an inner surface serving as a heat reflecting surface capable of reflecting radiation heat transmitted through the first heat reflector. In the case, it is possible that the second heat reflector has a mirror-finished inner surface.
Preferably, each of the heating elements has terminals protruded through the furnace body.
In addition, preferably, the resistance heat generating member has sections respectively having different cross sectional areas. Further preferably, the resistance heat generating member has sections respectively generating heat at different heat generating rates.
It is preferable that a space surrounding the reaction vessel is divided into a plurality of vertical regions, and that the heating elements are disposed in the plurality of vertical regions, respectively. Alternatively, it is preferable that a space surrounding the reaction vessel is divided into a plurality of circumferential regions, and that the heating elements are distributed in the plurality of circumferential regions. Alternatively, it is preferable that a space surrounding the reaction vessel is divided into a plurality of radial regions, and that the heating elements are distributed in the plurality of radial regions.
For example, the sealing member may be formed into a longitudinal shape, an U-like shape or a meandering shape.
For example, the resistance heat generating members may be a carbon wire that is formed by twisting strands of fine carbon members. The ceramics material may be for example quartz.
In addition, the thermal processing apparatus may comprise a holder capable of holding a plurality of objects to be processed in a tier-like manner and of being carried into the reaction vessel from below the reaction vessel.
In the case, preferably, the holder may have a lid capable of hermetically closing an open end of the reaction vessel, and a heat insulating unit disposed on the lid. In addition, it is preferable that a fourth heater is disposed in the heat insulating unit, and that the fourth heater includes heating elements, each having a sealing member made of a ceramic material, and a linear flexible resistance heat generating member sealed by the sealing member.